1. Field of the Invention
The present invention relates to a power supply device.
2. Description of Related Art
Power supply devices (switching regulator, LDO [low drop out] regulator and the like), which generate a desired output voltage from an input voltage and supply the output voltage to a load, are used in various kinds of applications.
As an output feedback method of a power supply device, voltage mode control is general, which detects a change in an output voltage by means of an error amplifier, a comparator or the like and, based on the detection result, controls power transistors (output transistor and synchronization rectification transistor). However, in the voltage mode control, a phase delay occurs because of an output capacitor; accordingly, there is a problem that a load response is low. In the meantime, if an output capacitor having large ESR [equivalent series resistance] is used, it is possible to reduce the phase delay and raise the load response. However, the larger the ESR of the output capacitor becomes, the larger the output ripple becomes; accordingly, there is a risk of impairing output stability.
Besides, as an output feedback method having a high load response, current mode control is known, which uses a ripple component of an inductor current to control a power transistor. In the meantime, as methods for detecting an inductor current, general methods include: a method which inserts a sense resistor in a route where an inductor current flows; and a method which uses on-resistance of a power transistor. However, there are problems that in the method which inserts a sense resistor, an unnecessary power loss occurs; and in the method which uses on-resistance of a power transistor, accuracy of detecting an inductor current deteriorates.
Besides, in both the voltage mode control and the current mode control, a cut-off frequency of an output feedback loop is curbed to about ⅕ to 1/10 of a switching frequency in light of stability of a system, and critical inductance resides in the system. Accordingly, there are problems that it is impossible to sharply change an inductor current and a long time is required until an output current reaches a desired value at a load sudden change.